PS to Brawlers Guild invites
However, others have done the same so it is going to be quite a job actually managing to get a clear shot at one and then be lucky enough to have an invite drop.
To repeat the information on that post, the NPCS that drop Brawlers invites are:
For Alliance players
Kar Warmaker - 68321 (appears just outside the eastern gates)
Muerta - 68322 (appears just near the graveyard)
Ubunti the Shade - 68320 (she appears inside the ground floor of the tower near the shore)
For Horde players
Dalan Nightbreaker - 68318
Disha Fearwarden - 68319
Mavis Harms - 68317
If you use the NPC Scan add on, you can add these NPCS to your scans (the numbers are their specific ID numbers so that you can include those into NPC Scan) so that if they appear the Add on lets you know while you are close to them. The Horde ones appear just outside the new Horde base while the Alliance ones are presumably similar and appear close to the Alliance base.
However, note, they dont drop the invite on every kill and I dont know at this time what their respawn rate is, although I suspect from a comment made while I was at the location that it is at least 30 mins.
Hope this all helps